In drilling wells for oil and gas exploration, understanding the structure and properties of the geological formation surrounding a borehole provides information to aid such exploration. However, the environment in which the drilling tools operate is often at a significant distance below the surface and measurements to manage operation of such equipment are made at these locations. Logging is the process of making measurements via sensors located downhole, which can provide valuable information regarding the formation characteristics. Measurement techniques can utilize electromagnetic signals that can make deep measurements, which are less affected by the borehole and the effects of the zone invaded by the drilling, and shallow measurements, which are near the tool providing the probe signals. Further, the usefulness of such measurements may be related to the precision or quality of the information derived from such measurements.
In electromagnetic sensing that can be applied to a borehole, imaging tools can achieve high azimuthal resolution, but cannot make deep measurements. On the other hand, some standard logging tools can achieve deep readings, but provide only limited azimuthal information. The main limitation can be related to the fact that dipoles are used in traditional induction logging tools. There are three types of methodologies that can be implemented to achieve azimuthal focusing: by positioning, by aperture, and by polarization.
Focusing by positioning can be achieved by placing the sensors in the vicinity of the area being sensed, for example, on a pad that can be made to contact a borehole wall. This is used in borehole imaging tools; however, their depth of investigation is limited in single well application, since depth of investigation is in the order of azimuthal resolution. Application of focusing by positioning is illustrated in FIG. 1 on the left of the vertical dotted line.
Focusing by aperture can be achieved by utilizing a special aperture such as a horn or a parabolic antenna. Although such system is very useful and can achieve very high azimuthal resolution in air, in a conductive formation it can lose its azimuthal focus at short distance from the aperture due to dispersive characteristics of the formation. Application of focusing by aperture is also shown in FIG. 1 on the left of the vertical dotted line.
Focusing by polarization, as used in current induction technology, can be deep but it can at most achieve three azimuthal modes, where only two of these are actively used, where an electromagnetic mode is a configuration, such as a field pattern, of an electromagnetic wave. This limitation is due to use of dipole wave phenomenon which limits the azimuthal information that can be gathered from deep inside the formation, as shown in FIG. 1 below the horizontal dotted line. As a result, obtaining high fidelity images deep within the formation by polarization has not been achieved.